Next-generation multimedia wireless communication systems under recent active study are required to additionally process and transmit various types of information including video data and radio data, beyond initial voice-oriented services. The wireless communication systems aim at reliable communication between a plurality of users irrespective of locations and mobility of the users. However, a wireless channel experiences a number of problems such as path loss, shadowing, fading, noise, limited bandwidth, limited power of terminals, and interference between different users. Other challenges faced in designing a wireless communication system include resource allocation, mobility issues related to fast changing physical channels, portability, security, and privacy.
When a transmission channel experiences deep fading, a receiver has difficulty in determining a transmitted signal unless another version or a replica of the transmitted signal is additionally transmitted. Resources corresponding to another version or a replica are called diversity and are one of the most significant factors contributing to reliable transmission on a wireless channel. Use of diversity can maximize data transmission capacity or data transmission reliability. A system that implements diversity by means of multiple transmit antennas and multiple receive antennas is called a multiple input multiple output (MIMO) system.
The MIMO system implements diversity by space frequency block code (SFBC), space time block code (STBC), cyclic delay diversity (CDD), frequency switched transmit diversity (FSTD), time switched transmit diversity (TSTD), precoding vector switching (PVS), spatial multiplexing (SM), etc.
FIG. 1 illustrates the configuration of a general MIMO communication system.
As illustrated in FIG. 1, if the numbers of transmit antennas and receive antennas are simultaneously increased to NT and NR, respectively, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case in which only either a transmitter or a receiver uses multiple antennas. Accordingly, it is possible to increase transmission rate and to remarkably improve frequency efficiency. Theoretically, the transmission rate according to an increase in channel transmission capacity can be increased by an amount obtained by multiplying an increase rate Ri indicated in the following Equation 1 by a maximum transmission rate Ro in case of using one antenna.
For example, in a MIMO communication system using four transmit antennas and four receive antennas, it is possible to theoretically obtain a transmission rate which is four times the transmission rate of a single antenna system. After an increase in the theoretical capacity of the MIMO system was first proved in the mid-1990s, various techniques for substantially improving data transmission rate have been actively developed. Several of these techniques have already been incorporated in a variety of wireless communication standards such as the 3rd generation mobile communication and the next-generation wireless local area network.R=min(NT,NR)  Equation 1
Active research up to now related to MIMO technology has focused on a number of different aspects, including research into information theory related to computation of MIMO communication capacity in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of a MIMO system, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
In a terminal structure having a general MIMO channel environment, a signal received by each receive antenna may be expressed as follows.
                    y        =                              [                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                                      [                                                                                                              h                          11                                                                                                                      h                          12                                                                                            …                                                                                              h                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    h                          21                                                                                                                      h                          22                                                                                            …                                                                                              h                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                h                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              h                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                                                    N                              R                                                        ⁢                            1                                                                                                                                                h                                                                                    N                              R                                                        ⁢                            2                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                              x                          1                                                                                                                                                              x                          2                                                                                                                                    ⋮                                                                                                                                      x                          j                                                                                                                                    ⋮                                                                                                                                      x                                                      N                            T                                                                                                                                ]                                            +                              [                                                                                                    n                        1                                                                                                                                                n                        2                                                                                                                        ⋮                                                                                                                          n                        i                                                                                                                        ⋮                                                                                                                          n                                                  N                          R                                                                                                                    ]                                      =                          Hx              +              n                                                          Equation        ⁢                                  ⁢        2            
A channel between transmit and receive antennas may be distinguished according to transmit and receive antenna indexes and a channel from transmit antenna j to receive antenna i is denoted by hij. When a precoding scheme is used as in LTE during transmission, transmission signal x may be expressed by:
                    ⁢          Equation      ⁢                          ⁢      3            x    =                  [                                                            x                1                                                                                        x                2                                                                        ⋮                                                                          x                i                                                                        ⋮                                                                          x                                  N                  T                                                                    ]            =                                    [                                                                                w                    11                                                                                        w                    12                                                                    …                                                                      w                                          1                      ⁢                                              N                        T                                                                                                                                                              w                    21                                                                                        w                    22                                                                    …                                                                      w                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          w                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            w                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      w                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          w                                                                  N                        T                                            ⁢                      1                                                                                                            w                                                                  N                        T                                            ⁢                      2                                                                                        …                                                                      w                                                                  N                        T                                            ⁢                                              N                        T                                                                                                                  ]                    ⁡                      [                                                                                                      s                      ^                                        1                                                                                                                                          s                      ^                                        2                                                                                                ⋮                                                                                                                        s                      ^                                        j                                                                                                ⋮                                                                                                                        s                      ^                                                              N                      T                                                                                            ]                          =                              W            ⁢                          s              ^                                =          WPs                    
where Wij of precoding matrix W denotes a weight between the i-th transmit antenna and the j-th Wo information. If transmission powers of transmitted signals are P1, P2, . . . , PNT, transmission information, transmission powers of which are adjusted, may be expressed by diagonal matrix P as follows.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    Equation        ⁢                                  ⁢        4            
Unlike this, a coordinated multi-point (COMP) scheme is technology in which data information is transmitted to one UE using transmit antennas of multiple cells having different PSCs and cell-specific pilot signals (reference signals), thereby enabling link selection and improving throughput/capacity caused by transmit diversity.
However, generally, when the quality of a radio link in a specific cell is degraded, the COMP scheme proposed up to now all disconnects currently used settings and performs resetting by searching for a serving cell regardless of the quality of the radio link.
The currently proposed scheme, however, maintains the quality of the radio link in a specific cell and, if the quality of the radio link of an SIR value of an RS of a specific cell is degraded makes a COMP agreement again using maintained cells by disconnecting the specific cell of COMP, thereby seamlessly transmitting data.
Meanwhile, one of systems considered beyond the 3rd generation is an orthogonal frequency division multiplexing (OFDM) system capable of attenuating an inter-symbol interference effect with low complexity. The OFDM system converts serially input data into N parallel data and transmits the parallel data on N orthogonal subcarriers. The subcarriers maintain frequency orthogonality. Orthogonal frequency division multiple access (OFDMA) is a multiple access scheme of achieving multiple access by independently providing a part of available subcarriers to each user in a system using OFDM as a modulation scheme.
FIG. 2 illustrates a wireless communication system.
Referring to FIG. 2, the wireless communication system includes at least one base station (BS) 20. Each BS 20 provides a communication service to a specific geographical area (generally called a cell) 20a, 20b, or 20c. Each cell may further be divided into a plurality of areas (called sectors). A user equipment (UE) 10 may be fixed or mobile. The term UE may be replaced with mobile station (MS), user terminal (UT), subscriber station (SS), wireless device, personal digital assistant (PDA), wireless modem, handheld device, etc. The BS 20 is generally a fixed station communicating with the UE 10 and the term BS is interchangeable with evolved Node B (eNB), base transceiver system (BTS), access point (AP), etc.
Downlink (DL) refers to communication from a BS to a UE and uplink (UL) refers to communication from a UE to a BS. A transmitter may be a part of a BS and a receiver may be a part of a UE, on DL, whereas the transmitter may be part of the UE and the receiver may be part of the BS, on UL.
The wireless communication system may be any of a MIMO system, a multiple input single output (MISO) system, a single input single output (SISO) system, and a single input multiple output (SIMO) system. The MIMO system uses a plurality of transmit antennas and a plurality of receive antennas. The MISO system uses a plurality of transmit antennas and a single receive antenna. The SISO system uses a single transmit antenna and a single receive antenna. The SIMO system uses a single transmit antenna and a plurality of receive antennas.
Hereinbelow, a transmit antenna refers to a physical or logical antenna used for transmitting one signal or stream and a receive antenna refers to a physical or logical antenna used for receiving one signal or stream.
A 3rd generation partnership project long term evolution (3GPP LTE) system employs MIMO. The LTE system will be described below in more detail.
FIG. 3 illustrates the structure of a radio frame in the 3GPP LTE system.
Referring to FIG. 3, a radio frame is divided into 10 subframes, each subframe including two slots. The slots of the radio frame are numbered from 0 to 19. A unit time during which one subframe is transmitted is called a transmission time interval (TTI). The TTI may be considered to be a scheduling unit for data transmission. For example, one radio frame may be 10 ms long, one subframe may be 1 ms long, and one slot may be 0.5 ms long.
This radio frame structure is purely exemplary and thus the number of subframes in a radio frame or the number of slots in a subframe may vary.
FIG. 4 illustrates the structure of a resource grid for one UL slot in the 3GPP LTE system.
Referring to FIG. 4, a UL slot includes a plurality of OFDM symbols in the time domain and NUL resource blocks (RBs) in the frequency domain. An OFDM symbol represents one symbol period, also called a single carrier-frequency division multiple access (SC-FDMA) symbol, an OFDMA symbol, or a symbol period according to systems. An RB is a resource allocation unit including a plurality of subcarriers in the frequency domain. The number of RBs included in a UL slot, NUL, depends on a UL bandwidth set for a cell. Each element of the resource grid is called a resource element (RE).
One RB includes 7×12 REs, that is, 7 OFDM symbols in the time domain by 12 subcarriers in the frequency domain, which is purely exemplary. Thus, the numbers of subcarriers and OFDM symbols in an RB are not limited to the above specific values. Rather, the number of OFDM symbols or the number of subcarriers in an RB may vary. The number of OFDM symbols may change according to a cyclic prefix (CP) length. For example, 7 OFDM symbols are included in an RB in the case of a normal CP, whereas 6 OFDM symbols are included in an RB in the case of an extended CP.
A resource grid for one UL slot in 3GPP LTE system of FIG. 4 may also be applied to the resource grid for a DL slot.
FIG. 5 illustrates the structure of a DL subframe.
The DL subframe includes two slots in the time slot, each slot including 7 OFDM symbols in the case of a normal CP. Up to three OFDM symbols (up to four OFDM symbols in a bandwidth of 1.4 Mhz) at the start of the first slot in the subframe are used for a control region to which control channels are allocated and the other OFDM symbols of the subframe are used for a data region to which a physical downlink shared channel (PDSCH) is allocated. The PDSCH is a channel on which a BS transmits data to a UE.
A physical downlink control channel (PDCCH) may deliver resource allocation information (a DL grant) and a transport format for a downlink shared channel (DL-SCH), resource allocation information (a UL grant) about an uplink shared channel (UL-SCH), paging information of a paging channel (PCH), system information on the DL-SCH, resource allocation information about a higher-layer control message such as a random access response transmitted on the PDSCH, a set of transmission power control (TPC) commands for individual UEs of a UE group, voice over Internet protocol (VoIP) activation information, etc. Control information transmitted on the above-described PDCCH is called downlink control information (DCI).
Now, a detailed description will be given of DL reference signals (RSs).
In the 3GPP LTE system, two types of DL RSs are defined for unicast services, a common RS or cell-specific RS (CRS) and a dedicated RS or UE-specific RS (DRS).
The CRS is an RS shared among all UEs within a cell, for use in acquisition of channel state information and handover measurement. The DRS is an RS specific to a UE, for use in data demodulation. Thus it can be said that CRS is a cell-specific RS and DRS is a UE-specific RS.
A UE measures CRSs and transmits feedback information such as channel quality information (CQI), a precoding matrix indicator (PMI), and a rank indicator (RI) to a BS. Then the BS performs DL frequency scheduling using the feedback information.
To transmit RSs to the UE, the BS allocates resources, in consideration of the amount of radio resources to be allocated to the RSs, exclusive positions of CRSs and DRSs, the positions of a synchronization channel (SCH) and a broadcast channel (BCH), and the density of the DRSs.
If a relatively large amount of resources are allocated to RSs, high channel estimation performance can be achieved but data rate is relatively decreased. On the other hand, if a relatively small amount of resources are allocated to RSs, high data rate can be achieved but the resultant low RS density may cause degradation of channel estimation performance. Accordingly, efficient resource allocation to RSs, considering channel estimation and data rate is an important factor to system performance.
Meanwhile, the DRS is used only for data demodulation, whereas the CRS is used for both channel information acquisition and data demodulation in the 3GPP LTE system. Especially, the CRS is transmitted in each subframe in a broadband, through each antenna port of a BS. For example, for two transmit antennas in the BS, CRSs are respectively transmitted through antenna port 0 and antenna port 1. For four transmit antennas in the BS, CRSs are respectively transmitted through antenna port 0 to antenna port 3.
FIG. 6 illustrates the structure of a UL subframe in the 3GPP LTE system.
Referring to FIG. 6, a UL subframe may be divided into a control region and a data region. A physical uplink control channel (PUCCH) including uplink control information is allocated to the control region. A physical uplink shared channel (PUSCH) including user data is allocated to the data region. In order to maintain a single carrier property, RBs allocated to one UE are contiguous in the frequency domain. That is, the UE cannot simultaneously transmit the PUCCH and the PUSCH.
The PUCCH for one UE is allocated to an RB pair in a subframe. The RBs of the RB pair occupy different subcarriers in first and second slots. Thus, the frequencies of the RBs of the RB pair allocated to the PUCCH are changed over a slot boundary. As the UE transmits UL control information on different subcarriers over time, a frequency diversity gain can be achieved.
UL control information transmitted on the PUCCH includes a hybrid automatic repeat request acknowledgement/negative acknowledgement (HARQ ACK/NACK), a channel quality indicator (CQI) indicating a DL channel state, and a scheduling request (SR) requesting UL radio resource allocation.
The PUSCH is mapped to a transport channel, uplink shared channel (UL-SCH). UL data transmitted on the PUSCH may be a transport block which is a data block for a UL-SCH transmitted during a TTI. The transport block may be user information. Alternatively, the UL data may be multiplexed data. The multiplexed data may be data obtained by multiplexing the transport block for the UL-SCH with control information. For example, the control information multiplexed for the data may include a CQI, a PMI, an HARQ ACK/NACK, an RI, etc. Alternatively, the UL data may include the control information only.
Meanwhile, a high data rate is required. The most basic and stable solution to the need for a high data rate is to increase bandwidth.
However, frequency resources are saturated at present and various techniques are partially used in a broad frequency band. To secure a broad bandwidth to satisfy higher data rate requirements for this reason, the concept of designing each of scattered bands so as to meet basic requirements for operating an independent system and aggregating a plurality of bands into one system has been introduced. This concept is called carrier aggregation (CA). Each independent operable band is defined as a component carrier (CC).
CA is adopted in an LTE-advanced (LTE-A) system as well as in the LTE system.
Carrier Aggregation
A CA system is a wireless communication system that configures a desired broad band by aggregating one or more carriers each having a narrower bandwidth than a target broad band. The CA system is also called a multiple carrier system, a bandwidth aggregation system, etc. CA systems may be categorized into a contiguous CA system using contiguous carriers and a non-contiguous CA system using non-contiguous carriers. It should be understood that a multi-carrier system or a CA system covers both a contiguous CC case and a non-contiguous CC case in the following description.
A guard band may be interposed between carriers in the contiguous CA system. To ensure backward compatibility with a legacy system, each of one or more carriers that are aggregated may use bandwidth defined in the legacy system. For example, the 3GPP LTE system supports 1.4, 3, 5, 10, 15 and 20 MHz. Alternatively, a broad band may be configured by defining a new bandwidth, instead of using the bandwidths of the legacy system.
A UE may simultaneously transmit or receive one or more carriers according to the capacity thereof in the CA system.
FIG. 7 illustrates an example of communication on a single CC. This communication may be conducted in the LTE system.
Referring to FIG. 7, data transmission and reception are performed in a single DL band and a single UL band corresponding to the DL band in a typical frequency division duplex (FDD) wireless communication system. A BS and a UE transmit and receive data and/or control information that is scheduled in the units of subframes. The data is transmitted and received in the data region configured in a UL/DL subframe and the control information is transmitted and received in the control region configured in the UL/DL subframe. For transmission and reception of the data and control information, the UL/DL subframe carries signals on various physical channels. While FIG. 7 is described mainly in the context of FDD, the same description is also applicable to a time division duplex (TDD) scheme in which a radio frame is divided into UL and DL in the time domain.
FIG. 8 illustrates an example of communication on multiple CCs.
FIG. 8 may correspond to an example of communication in the LTE-A system.
The LTE-A system uses CA, bandwidth aggregation, or spectrum aggregation technology by collecting a plurality of UL/DL frequency blocks to use a broader frequency band. Each frequency block is transmitted on a CC. In this disclosure, a CC may refer to a frequency block for CA or the center subcarrier of the frequency block. These two meanings are used interchangeably.
On the other hand, the 3GPP LTE system supports different configurations of a DL bandwidth and a UL bandwidth based on one CC. Although the 3GPP LTE system supports a maximum of 20 MHz and different configurations of a UL bandwidth and a DL bandwidth, only one CC is supported for each of UL and DL.
However, spectrum aggregation (also called bandwidth aggregation or carrier aggregation) supports a plurality of CCs. For example, if five CCs are allocated as granularity of each carrier having a bandwidth of 20 MHz, up to a bandwidth of 100 MHz can be supported.
A pair of one DL CC or UL CC and one DL CC may correspond to one cell. One cell basically includes one DL CC and optionally includes a UL CC. Accordingly, it may be said that a UE communicating with a BS through a plurality of DL CCs is provided with a service by a plurality of serving cells. In this case, a plurality of DL CCs may be configured in DL and only one CC may be used in UL. Then, it can be said that the UE is provided a service by a plurality of serving cells in DL and is provided a service only by one serving cell in UL.
In this meaning, serving cells may be divided into a primary cell and a secondary cell. The primary cell operates in a primary frequency and is a cell designated as a primary cell when a UE performs an initial connection establishment process, initiates a connection re-establishment process, or performs a handover process. The primary cell is also referred to as a reference cell. The secondary cell operates in a secondary frequency, may be configured after RRC connection is established, and may be used to provide an additional radio resource. At least one primary cell may always be set and a secondary cell may be added/modified/released by higher layer signaling (e.g. an RRC message).
Referring to FIG. 8, a bandwidth of 100 MHz may be supported by aggregating five 20-MHz CCs on UL/DL. The CCs may be contiguous or non-contiguous in the frequency domain. For convenience, the bandwidth of a UL CC and the bandwidth of a DL CC in FIG. 9 are identically and symmetrically illustrated. However, the bandwidth of each CC may be independently determined. For example, the bandwidths of UL CCs may be configured into 5 MHz (UL CC0)+20 MHz (UL CC1)+20 MHz (UL CC2)+20 MHz (UL CC3)+5 MHz (UL CC4). In addition, asymmetrical CA is also possible by configuring different numbers of UL CCs and DL CCs. Asymmetrical CA may occur due to a limited available frequency band or may be artificially implemented according to network setting. For example, despite the existence of N CCs in a total system band, a frequency band that a specific UE can receive may be limited to M (<N) CCs. Various CA parameters may be configured cell-specifically, UE group-specifically, or UE-specifically.
While a UL signal and a DL signal are transmitted on one-to-one mapped CCs in the illustrated case of FIG. 8 by way of example, the number of actual CCs carrying signals may vary depending on network setting or the type of the signals.
For instance, when a scheduling command is transmitted through DL CC1 on DL, data corresponding to the scheduling command may be transmitted through another DL CC or a UL CC. In addition, control information related to a DL CC may be transmitted through a specific UL CC on UL irrespective of DL-UL CC mapping. DCI may be transmitted through a specific DL CC in a similar manner.
FIG. 9 is a block diagram for explaining an SC-FDMA transmission scheme which is a UL access scheme adopted in the 3GPP LTE system.
For LTE UL, SC-FDMA is adopted, which is similar to OFDM but reduces the power consumption and power amplifier cost of a portable terminal by reducing a peak to average power ratio (PAPR).
SC-FDMA is very similar to OFDM in that a signal is transmitted on subcarriers by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). SC-FDMA is also similar to OFDM in that a simple equalizer can be used in the frequency domain by using a guard interval (CP) against inter-symbol interference (ISI) caused by multi-path fading. However, compared to OFDM, SC-FDMA improves the power efficiency of a transmitter by reducing the PAPR of the transmitter by about 2 to 3 dB through additional unique techniques.
A problem encountered with a conventional OFDM transmitter is that signals on subcarriers along the frequency axis are converted into signals on the time axis by IFFT. Since IFFT is a process of parallel execution of the same operation, IFFT increases PAPR.
Referring to FIG. 9, in SC-FDMA, information is first subjected to discrete Fourier transform (DFT) 102 before signals are mapped to subcarriers to solve the above problem. The DFT-spread signals (or DFT-precoded signals) are mapped to subcarriers through subcarrier mapping 13 and converted into signals into the time axis through IFFT 14.
SC-FDMA is advantageous for transmission power efficiency because the PAPR of a time-domain signal after IFFT 14 is not greatly increased as opposed to OFDM due to the correlation among DFT 12, subcarrier mapping 13, and IFFT 14.
The transmission scheme of performing IFFT after DFT spreading is called SC-FDMA.
SC-FDMA is advantageous in that it is robust against multi-path fading channels due to a similar structure to OFDM and the problem of a PAPR increase encountered with OFDM is completely solved through IFFT. Consequently, the power amplifier can be efficiently used. Meanwhile, SC-FDMA is also called DFT spread OFDM (DFT-s-OFDM).
That is, SC-FDMA can reduce a PAPR or cubic metric (CM). Furthermore, the non-linear distortion range of the power amplifier can be avoided by using an SC-FDMA transmission scheme, thereby increasing the transmission power efficiency of a UE having limited power consumption. Accordingly, user throughput can be increased.
3GPP is actively working on standardization of LTE-A evolved from LTE. Although SC-FDMA-based techniques competed with OFDM-based techniques as in the standardization process of LTE, clustered-DFT-s-OFDM which allows non-contiguous resource allocation has been adopted.
The LTE-A system will be described below in detail.
FIG. 10 is a block diagram for explaining clustered DFT-s-OFDM adopted as a UL access scheme in the LTE-A standard.
The main feature of a clustered DFT-s-OFDM scheme is that DFT-s-OFDM can flexibly cope with a frequency selective fading environment by enabling frequency selective resource allocation.
Compared to SC-FDMA which is the conventional LTE UL access scheme, clustered DFT-s-OFDM adopted as an LTE-A UL access scheme allows non-contiguous resource allocation. Therefore, UL transmission data may be partitioned into a plurality of clusters.
That is, while the LTE system maintains the single carrier property for UL, the LTE-A system allows non-contiguous allocation of DFT-precoded data on the frequency axis or simultaneous transmission of a PUSCH and a PUCCH.